EPROMS utilize memory elements which rely on avalanche injection of electrons to an electrically isolated floating gate in order to program the device. The charge stored on this floating gate may be removed by exposing it to UV light. In some applications there is a need for a one-time programmable EPROM the programmed condition of which is unaffected by UV light. Examples of such potential uses include redundancy circuits, custom reconfiguration and parameter storage. One attempt at providing such a device is disclosed in an article by Alan C. Folmsbee in the 1983 edition of IEDM at page 574 entitled "PROM Cell Made With An EPROM Process". The Folmsbee device consists of a double polycrystalline silicon floating-gate EPROM cell that employs a metal shield. The shield contacts the substrate adjacent the floating gates and extends over the floating gates and therebeyond a distance of the order of 80 micrometers overlying an elongated thin wafer of silicon dioxide. A second metal contact adjacent the elongated end of the shield has a cap portion extending toward the former end to define a narrow shield opening. UV light entering the silicon dioxide wafer through the shield opening undergoes refraction according to Snell's law so that all rays will travel at an angle of less than 42 to the perpendicular and undergo a number of reflections as they travel toward the floating gates. With each reflection from the silicon substrate a significant fraction of the light is absorbed thereby attenuating the UV intensity. Obviously, the Folmsbee device does not eliminate the effect of UV light but merely reduces its effect by an amount which is approximately proportional to the length of the shield extension. However, a long shield is inconsistent with a need to maximize the density of EPROM cells on a substrate.
It is therefore a principal object of this invention to provide an improved electrically programmable floating gate MOS device. Another object is to provide a floating gate EPROM which has an improved resistance to erasure by UV radiation.